Electric enhanced transmission for multi-spool load-sharing turbofan engine

ABSTRACT

A turbofan engine includes a first spool including a first turbine, and a first tower shaft engaged to the first spool. A second spool includes a second turbine, and a second tower shaft is engaged to the second spool. A superposition gearbox includes a sun gear, a plurality of intermediate gears engaged to the sun gear, and is supported in a carrier and a ring gear circumscribing the intermediate gears. The first tower shaft or the second tower shaft drives one of the intermediate gears. A drive motor is engaged to drive the sun gear, an inner electric motor, a stator disposed radially outside of the inner electric motor, and an outer electric motor disposed radially outside the stator. A first load on the first spool and a second load on the second spool is adjusted by operation of at least one of the inner electric motor and the outer electric motor.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section and a turbine section. Air entering thecompressor section is compressed and delivered into the combustionsection where it is mixed with fuel and ignited to generate ahigh-energy exhaust gas flow. The high-energy exhaust gas flow expandsthrough the turbine section to drive the compressor and the fan section.The compressor section typically includes low and high pressurecompressors, and the turbine section includes low and high pressureturbines.

Incorporation of electric power in gas turbine engines is currentlysubstantially limited to accessory components. Advances in electricmotors and generators along with demands for ever increasing engineoperating efficiencies warrant consideration of alternate engineconfigurations.

Turbine engine manufacturers continue to seek improvements to engineperformance including improvements to propulsive efficiencies.

SUMMARY

A turbofan engine according to an exemplary embodiment of thisdisclosure, includes, among other possible things, a first spoolincluding a first turbine and a first tower shaft engaged to the firstspool. A second spool includes a second turbine, and a second towershaft is engaged to the second spool. A superposition gearbox includes asun gear, a plurality of intermediate gears engaged to the sun gear, andis supported in a carrier and a ring gear circumscribing theintermediate gears. One of the first tower shaft or the second towershaft drives one of the intermediate gears. A drive motor is engaged todrive the sun gear, an inner electric motor, a stator disposed radiallyoutside of the inner electric motor; and an outer electric motordisposed radially outside the stator. A first load on the first spooland a second load on the second spool is adjusted by operation of atleast one of the inner electric motor and the outer electric motor.

In a further embodiment of the foregoing turbofan engine, the carrier isfixed to a static structure of the turbofan engine.

In a further embodiment of any of the foregoing turbofan engines, thefirst tower shaft and the second tower shaft are disposed within acommon radial plane.

In a further embodiment of any of the foregoing turbofan engines, thestator is fixed to the static structure of the turbofan engine.

In a further embodiment of any of the foregoing turbofan engines, thefirst tower shaft is engaged to drive the inner electric motor and thering gear is engaged to drive the outer electric motor.

In a further embodiment of any of the foregoing turbofan engines, thesecond tower shaft is engaged to drive the inner electric motor and thering gear is engaged to drive the outer electric motor.

In a further embodiment of any of the foregoing turbofan engines, thefirst spool includes a first compressor section coupled to a firstturbine section. The first tower shaft and the second tower shaft aredisposed forward of the first compressor section.

In a further embodiment of any of the foregoing turbofan engines, thesecond spool includes a second compressor section that is disposedforward of the first tower shaft and the second tower shaft.

In a further embodiment of any of the foregoing turbofan engines, acontroller is in electric communication with the drive motor, the innerelectric motor and the outer electric motor. The controller proportionsthe first load and the second load between the first spool and thesecond spool by controlling a speed of the drive motor and a statorfield of the inner electric motor and the outer electric motor.

Another turbofan engine according to an exemplary embodiment of thisdisclosure includes, among other possible things, a first spoolincluding a first turbine and a second spool including a second turbine.A superposition gearbox is disposed about an axis and includes a sungear, a plurality of intermediate gears engaged to the sun gear andsupported in a carrier and a ring gear circumscribing the intermediategears. A drive means is engaged to drive the sun gear. An integratedmotor generator is disposed about the axis and includes an innerarmature and an outer armature rotatable relative to a fixed statordisposed radially between the inner armature and the outer armature. Oneof the inner armature and the outer armature is mechanically coupled tothe first spool and the other of the inner armature and the outerarmature is coupled to a portion of the superposition gearbox. A firstload on the first spool and a second load on the second spool isadjusted by modification of stator fields of the inner armature and theouter armature.

In a further embodiment of the foregoing turbofan engine, the carrier isfixed to the static structure of the turbofan engine.

In another embodiment of the any of the foregoing turbofan engines, afirst tower shaft is engaged to be driven by the first spool and asecond tower shaft is engaged to be driven by the second spool. The oneof the first tower shaft and the second tower shaft drive one of theintermediate gears and the other of the first tower shaft and the secondtower shaft drive one of the inner armature or the outer armature.

In another embodiment of the any of the foregoing turbofan engines, thedrive means comprises an electric motor.

In another embodiment of the any of the foregoing turbofan engines, acontroller is in electric communication with the drive motor and theintegrated motor generator for proportioning the first load and thesecond load between the first spool and the second spool, by controllinga speed of the drive motor and stator fields of the inner armature andthe outer armature.

A method of proportioning a load between spools of a turbofan engineaccording to an exemplary embodiment of this disclosure includes, amongother possible things, coupling an intermediate gear of a superpositiongearbox to a first spool. The superposition gearbox includes a sun gear,a plurality of intermediate gears and a ring gear circumscribing theplurality of the intermediate gears. A first portion of an integratedmotor generator is coupled to a second spool. A second portion of theintegrated motor generator is coupled to the superposition gearbox. Thesun gear is coupled to a drive means, controlling a speed of the sungear with the drive means. A stator field of the integrated motorgenerator is controlled to proportion a load between the first spool andthe second spool.

In a further embodiment of the foregoing method of proportioning a loadbetween spools of a turbofan engine, the first spool drives with theintegrated motor/generator for starting the turbofan engine.

In a further embodiment of any of the foregoing methods of proportioninga load between spools of a turbofan engine, the carrier is fixed to astatic structure of the turbofan engine and the drive means comprises anelectric motor driving the sun gear of the superposition gearbox.

In a further embodiment of any of the foregoing methods of proportioninga load between spools of a turbofan engine, the integrated motorgenerator includes an inner armature and an outer armature disposedbetween a fixed stator and controlling a load between the first spooland the second spool. A stator field is adjusted between each of theinner armature and the outer armature.

Although the different examples have the specific components shown inthe illustrations, embodiments of this invention are not limited tothose particular combinations. It is possible to use some of thecomponents or features from one of the examples in combination withfeatures or components from another one of the examples.

These and other features disclosed herein can be best understood fromthe following specification and drawings, the following of which is abrief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example gas turbine engine.

FIG. 2 is a schematic view of a gas turbine engine including an electricenhanced transmission system for multi-spool load sharing.

FIG. 3 is a schematic view of a portion of an example superpositiongearbox embodiment of the system.

FIG. 4 is a schematic view of another portion of the examplesuperposition gearbox embodiment.

FIG. 5 is a schematic view of an example integrated generator motorembodiment of the system.

FIG. 6 is a block diagram of an example control system for the enhancedtransmission system.

FIG. 7 is a schematic view of another turbofan engine including anotherexample electric enhanced transmission system embodiment.

FIG. 8 is another turbofan engine including another example electricenhanced transmission system embodiment.

FIG. 9 is yet another example turbofan engine with another exampleenhanced transmission system embodiment.

FIG. 10 is a schematic view of a portion of another integrated motorgenerator embodiment.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 drivesair along a bypass flow path B in a bypass duct defined within a nacelle18, and also drives air along a core flow path C for compression andcommunication into the combustor section 26 then expansion through theturbine section 28. Although depicted as a two-spool turbofan gasturbine engine in the disclosed non-limiting embodiment, it should beunderstood that the concepts described herein are not limited to usewith two-spool turbofans as the teachings may be applied to other typesof turbine engines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a low pressure compressor 44 and a low pressure turbine46. The inner shaft 40 is connected to a fan section 22 through a speedchange mechanism, which in exemplary gas turbine engine 20 isillustrated as a geared architecture 48 to drive fan blades 42 at alower speed than the low speed spool 30. The high speed spool 32includes an outer shaft 50 that interconnects a high pressure compressor52 and a high pressure turbine 54. A combustor 56 is arranged inexemplary gas turbine 20 between the high pressure compressor 52 and thehigh pressure turbine 54. A mid-turbine frame 58 of the engine staticstructure 36 may be arranged generally between the high pressure turbine54 and the low pressure turbine 46. The mid-turbine frame 58 furthersupports bearing systems 38 in the turbine section 28. The inner shaft40 and the outer shaft 50 are concentric and rotate via bearing systems38 about the engine central longitudinal axis A which is collinear withtheir longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 58 includes airfoils 60 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of the low pressure compressor 44 andthe fan blades 42 may be positioned forward or aft of the location ofthe geared architecture 48 or even aft of turbine section 28.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1 and less than about 5:1. Itshould be understood, however, that the above parameters are onlyexemplary of one embodiment of a geared architecture engine.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and35,000 ft (10,668 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (‘TSFC’)”—is the industry standard parameter of lbm of fuelbeing burned divided by lbf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram° R)/(518.7° R)]^(0.5). The “Lowcorrected fan tip speed” as disclosed herein according to onenon-limiting embodiment is less than about 1150 ft/second (350.5meters/second). The fan tip speed is measured at one of the bucketcruise and take-off operating conditions of the engine 20.

The example gas turbine engine includes the fan section 22 thatcomprises in one non-limiting embodiment less than about 26 fan blades42. In another non-limiting embodiment, the fan section 22 includes lessthan about 20 fan blades 42. Moreover, in one disclosed embodiment thelow pressure turbine 46 includes no more than about 6 turbine rotorsschematically indicated at 34. In another non-limiting exampleembodiment, the low pressure turbine 46 includes about 3 turbine rotors.A ratio between the number of fan blades 42 and the number of lowpressure turbine rotors is between about 3.3 and about 8.6. The examplelow pressure turbine 46 provides the driving power to rotate the fansection 22 and therefore the relationship between the number of turbinerotors 34 in the low pressure turbine 46 and the number of blades 42 inthe fan section 22 disclose an example gas turbine engine 20 withincreased power transfer efficiency.

Changes in environmental conditions can require constant adaptations andadjustments to engine operation to provide a desired propulsive output.For example, fuel flow to the combustor 56 may be adjusted dependingboth on a desired propulsive power output and input airflowcharacteristics including pressure and temperatures. Changes in inputairflows may change during operation and require adjustment of fuel flowto maintain the desired propulsive output. There is a certain lagbetween the adjustment and obtaining the operating propulsive output.Although very brief, the lag can affect engine efficiency.

Moreover, changes in power provided by the low pressure turbine 46driving fan section 22 also will add power to the low pressurecompressor 44 and thereby complicate operation. The low pressurecompressor 44 matches operation to that of the high pressure compressor52 and thereby any adjustment to one results in changes to the other.Excessive power input into the low pressure compressor 44 may requirethat air flow be bled off in order to properly match operation of thehigh pressure compressor 52.

The example gas turbine engine 20 includes an enhanced electrictransmission system 62 that enables load sharing between the spools 30,32. The example transmission system 62 includes a superposition gearbox70 that is driven by one of a first tower shaft 66 coupled to the highspeed spool 32 and a second tower shaft 68 coupled to the low speedspool 30. An integrated motor/generator 72 is coupled to thesuperposition gearbox 70. An aircraft controller schematically indicatedat 64 controls operation of the superposition gearbox 70 and themotor/generator 72 in concert with a drive motor 74 to distribute loadsbetween the low speed spool 30 and the high speed spool 32.

In this disclosed embodiment, the first tower shaft 66 and the secondtower shaft 68 are disposed within a common radial plane schematicallyillustrated at 16. It should be appreciated that the tower shafts 66 and68 may be coupled in other manners and disposed within different planesas is required for a specific engine configuration. Moreover, the spools30, 32 may be co-rotating or counter-rotating depending on the engineconfiguration and remain within the contemplation of this disclosure.

Referring to FIG. 2, the example electric enhanced transmission system62 is schematically shown with the superposition gearbox 70 and themotor/generator 72 coupled to the engine 20. The motor/generator 72 iscoupled to portions of the superposition gearbox 70 and also is drivenby one of the first and second tower shafts 66, 68.

The disclosed superposition gearbox 70 is an epicyclic gearbox thatincludes a sun gear 76 that rotates about an axis 75 and drivesintermediate gears 80 supported within a carrier 82. In this example,the carrier 82 is fixed to the static structure 36 of the turbofanengine 20. The fixed carrier 82 maintains the axes of rotation for eachof the intermediate gears 80. A ring gear 84 is disposed about theplurality of intermediate gears 80 and is free to rotate about the axis75 in response to driving engagement by the intermediate gears 80.

A drive means 74 is coupled to the sun gear 76 through a sun shaft 78.In this example embodiment, the drive means 74 is an electric motor 74.The electric motor 74 therefore drives the sun gear 76 of thesuperposition gearbox 70. In this disclosed example illustrated in FIG.2, the second tower shaft 68 coupled to the low speed spool 30 isengaged to drive one of the intermediate gears 80 supported within acarrier 82. The other intermediate gears 80 operate as conventionallyutilized in an epicyclic gear system. Accordingly, the superpositiongearbox 70 includes a first drive input from the electric motor 74 todrive the sun gear 76 and a second drive input from the second towershaft 68 to drive one of the intermediate gears 80. The ring gear 84 iscoupled to a portion of the integrated motor generator 72 to provide athird drive input to the gearbox 70.

Referring to FIGS. 3, 4 and 5 with continued reference to FIG. 2, themotor/generator 72 includes a stator 90 that is fixed to a staticstructure 36. An inner armature 86 is disposed radially inward of thestator 90 and an outer armature 88 is disposed radially outward of thestator 90. The inner armature 86 and the outer armature 88 are annularstructures that rotate relative to the fixed stator 90. The stator 90 iselectrically coupled to both the inner and outer armatures 86, 88. Theouter armature 88, along with the stator 90, defines an outermotor/generator 94. The inner armature 86 along with the same stator 90defines an inner motor/generator 92. The inner armature 86 and the outerarmature 88 can be permanent magnets or conventional windings.

The example superposition gearbox 70 includes a first input from thesecond tower shaft 68 from the low speed spool 30 and a second input tothe sun gear 76 by the electric motor 74. The first tower shaft 66extends through the superposition gearbox 70 along the axis 75 to themotor/generator and is not mechanically coupled to the gearbox 70.

The integrated motor generator 72 is driven in part by the ring gear 84from the superposition gearbox 70 and by the first tower shaft 66.Alternatively, the motor/generator 72 can be operated as an electricmotor to drive the first tower shaft 66. In this disclosed embodiment,the ring gear 84 drives the outer armature 88 relative to the staticstator 90. The first tower shaft 66 drives the inner armature 86relative to the same stator 90. It should be appreciated that althoughthe first tower shaft 66 is shown engaged to the inner armature 86 thatthe inputs to the motor/generator 72 may be modified such that thesecond tower shaft 68 driven by the low speed spool 30 could beconfigured to drive the inner armature 86 instead of the first towershaft 66. Similarly, the intermediate gear 80 could be alternativelydriven by the first tower shaft 66 instead of the second tower shaft 68as shown in FIG. 2.

The inner armature 86 of the inner motor/generator 92 includes aplurality of poles 85. The outer armature 88 of the outer/motorgenerator 94 includes a plurality of poles 87. The stator is configuredto correspond with the number of poles 85, 87 within each of the innerand outer motor/generators 92, 94. The number of poles 85, 87 along withthe configuration of the stator 90 is determined in view of the speedrelationships provided by the superposition gearbox 70 along with therelative speeds of the low speed and high speed spools 30, 32.Additionally, although an example motor/generator configuration isdisclosed, other electric motor configurations could also be utilizedand are within the contemplation and scope of this disclosure.

Referring to FIG. 6 with continued reference to FIGS. 2-5, an examplecontrol system 95 includes the FADEC 64 that receives information froman aircraft control system 96. The aircraft control system 96 is also incommunication and controls an aircraft electric power network 98.Electric power from the aircraft electric power network 98 can beprovided to the drive motor 74 to provide control over operation of thesuperposition gearbox 70. Alternatively, electric power from themotor/generator 72 may be provided for operation of the drive motor 74.The example control system 95 includes a motor controller 100 thatreceives information from the engine FADEC 64. The FADEC also controls amotor controller 102 in communication with the motor/generator 72. Themotor/generator control 102 controls operation of the stator 90 alongwith a stator field control 104. It should be appreciated, that thecontrol system 95 may be of different configurations and implemented asa separate system or as part of the overall engine and/or aircraftcontrol system. Moreover, the control system 95 would be modified toaccommodate alternate disclosed system embodiments including differentinput configurations.

The transmission system 62 integrates the superposition gearbox 70 withthe motor/generator 72 to facilitate the proportioning and sharing ofloads across the high speed spool 32 and the low speed spool 30. Inoperation, the inner motor/generator 92 is driven by the first towershaft 66 coupled to the high speed spool 32. The outer/generator 94 isdriven by the superposition gearbox 70 through the ring gear 84. Thestator 90 disposed between the inner motor/generator 92 and the outermotor/generator 94 is controlled to manipulate a field therebetween thatis utilized to transfer loads between the low speed spool 30 and thehigh speed spool 32. The transfer of loads between the spools 30, 32 istransferred through a combination of power input and output. Operationof the motor/generators 92,94 enables power to be drawn from one spooland input into the other spool through the interface provided betweenthe inner and outer motor/generators 92, 94.

The stator field control 104 changes the rate of rotation of the statorfield about the axis 75 and the slip of the electro-magnetic field ofthe motor/generator 92 versus the electro-magnetic field ofmotor/generator 94. The electric motor 74 supplements the powervariations of the varying slip. The transfer of loads enhances the ratesof change of the rotational speeds of low speed spool 30 and high speedspool 32, for decelerations and accelerations of the spools.

In operation during a power increase of engine 20 as achieved byincreasing the rate of fuel flow combusted in combustor 56, there can bean excessive acceleration of the rotational speed of high speed spool 32and a lag in the acceleration of the rotational speed of the low speedspool 30 versus the rotational speed of the high speed spool 32. Theacceleration of the low speed spool 30 can be controllably enhanced bydrawing power from the high speed spool 32 by the inner motor-generator92 and transferring the power to the outer motor generator 94. The powertransferred to the low speed spool 30 by the motor generator 94increases the rate of acceleration of the low speed spool 30 and reducesthe lag between the accelerations of the spools. This reduces the timerequired to increase the power output of the engine 20 as criticalduring a go-around abort of landing an aircraft.

In operation during a power decrease of engine 20 as achieved bydecreasing the rate of fuel flow combusted in combustor 56, there can bean insufficient deceleration of the rotational speed of low speed spool30 and a lag in the deceleration of the rotational speed of the lowspeed spool 30 versus high speed spool 32. The deceleration of low speedspool 30 can be can be controllably enhanced by drawing power from thelow speed spool 30 by the outer motor-generator 94 and transferring thepower to the inner motor generator 92. The power transferred from thelow speed spool 30 by the motor generator 94 increases the rate ofdeceleration of the low speed spool 30 and reduces the lag betweenspools. This reduces the time required to decrease the power output ofengine 20 as critical in a steep approach for landing an aircraft.Smaller and more frequent changes in the power output of engine 20 asoccurring during the cruise segment of an aircraft mission can beachieved similarly by the transfer of loads, but without a change in therate of fuel flow combusted in combustor 56, by controlling motorgenerators 92, 92 with motor generator control 102 and controllingsuperposition motor 74 with motor control 100.

The transfer of loads enabled by superposition gearbox 70 and themotor/generators 92, 94 and the aircraft electric power network 98 andelectric motor 74 enhance the safety of operation of multi-engineaircraft. There are operating points of altitude and flight speed thatpreclude a windmill starting of an engine due to inadequate wind millingof the low spool 30 by the fan 42. In the event two engines 20, 20 mustbe windmill started nearly simultaneously at these operating points, thewind milling of one fan 42 of engine 20 can transfer sufficient powerthrough the aircraft electric power network 98 to a second wind millingengine 20 to achieve a start of the second engine.

Referring to FIG. 7, with continued reference to FIGS. 3-5, the examplegas turbofan engine 20 is schematically shown with an alternateconfiguration of the enhanced electric transmission system 62′. Thesystem 62′ includes the second tower shaft 68 extending through thesuperposition gearbox 70′ and coupled to drive the inner armature 86 ofthe motor generator 72′. The first tower shaft 66 extends into thesuperposition gearbox 70′ and drives the intermediate gear 80.

Referring to FIGS. 8, 9 and 10 alternate embodiments of the enhancedtransmission system 110 and 110′ illustrate additional examples of thesuperposition gearbox 108,108′ and the motor/generator 106, 106′. In thedisclosed embodiments 110, 110′ the ring gear 84 drives the innerarmature 86 and one of the first and second tower shafts 66, 68 drivesthe outer armature 88. FIG. 8 illustrates an embodiment of thetransmission system 110 with the first tower shaft 66 coupling the highspool 32 to the outer armature 88. FIG. 9 illustrates an embodiment ofthe transmission system 110′ with the second tower shaft 68 coupling thelow spool 30 to the outer armature 88. It should be appreciated that thefirst and second tower shafts 66, 68 may be alternated between drivingportions of the example motor/generators 106, 106′ and portions of thesuperposition gearbox 108, 108′.

FIG. 10 illustrates the motor/generator 106, 106′ with the outerarmature 88 driven by one of the tower shafts 66, 68 and the innerarmature 86 driven by the ring gear 84. The configurations of respectiveinner and outer armatures 86, 88 are modified to correspond with anycorresponding differences in relative speeds necessary to implementoperation of the motor/generator 106, 106′.

The integrated arrangement of superposition gearbox 70 with the innerand outer motor/generators 92,94 in combination with the drive motor 74enables manipulation and control of load proportioning and sharingbetween the high and low spools.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of this disclosure. For that reason, the followingclaims should be studied to determine the scope and content of thisdisclosure.

What is claimed is:
 1. A turbofan engine comprising: a first spoolincluding a first turbine; a first tower shaft engaged to the firstspool; a second spool including a second turbine; a second tower shaftengaged to the second spool; a superposition gearbox including a sungear, a plurality of intermediate gears engaged to the sun gear andsupported in a carrier and a ring gear circumscribing the intermediategears, wherein one of the first tower shaft or the second tower shaftdrives one of the intermediate gears; a drive motor engaged to drive thesun gear; an inner motor/generator including an inner armature and astator disposed radially outside of the inner armature; and an outermotor/generator including the stator and an outer armature disposedradially outside the stator, wherein a first load on the first spool anda second load on the second spool is adjusted by operation of at leastone of the inner electric motor and the outer electric motor.
 2. Theturbofan engine as recited in claim 1, wherein the carrier is fixed to astatic structure of the turbofan engine.
 3. The turbofan engine asrecited in claim 2, wherein the first tower shaft and the second towershaft are disposed within a common radial plane.
 4. The turbofan engineas recited as claim 1, wherein the stator is fixed to the staticstructure of the turbofan engine.
 5. The turbofan engine as recited inclaim 4, wherein the first tower shaft is engaged to drive the innermotor/generator and the ring gear is engaged to drive the outermotor/generator.
 6. The turbofan engine as recited in claim 4, whereinthe second tower shaft is engaged to drive the inner motor/generator andthe ring gear is engaged to drive the outer motor/generator.
 7. Theturbofan engine as recited in claim 1, wherein first spool includes afirst compressor section coupled to a first turbine section, the firsttower shaft and the second tower shaft are disposed forward of the firstcompressor section.
 8. The turbofan engine as recited in claim 7,wherein the second spool includes a second compressor section that isdisposed forward of the first tower shaft and the second tower shaft. 9.The turbofan engine as recited in claim 1, including a controller inelectric communication with the drive motor, the inner motor/generatorand the outer motor/generator for proportioning the first load and thesecond load between the first spool and the second spool by controllinga speed of the drive motor and a stator field of the innermotor/generator and the outer motor/generator.
 10. A turbofan enginecomprising: a first spool including a first turbine; a second spoolincluding a second turbine; a superposition gearbox disposed about anaxis and including a sun gear, a plurality of intermediate gears engagedto the sun gear and supported in a carrier and a ring gearcircumscribing the intermediate gears, wherein the carrier is fixed to astatic structure of the turbofan engine; a drive means engaged to drivethe sun gear; an integrated motor generator disposed about the axis andincluding an inner armature and an outer armature rotatable relative toa fixed stator disposed radially between the inner armature and theouter armature, wherein one of the inner armature and the outer armatureis mechanically coupled to the first spool and the other of the innerarmature and the outer armature is coupled to a portion of thesuperposition gearbox and a first load on the first spool and a secondload on the second spool is adjusted by modification of stator fields ofthe inner armature and the outer armature; a first tower shaft engagedto be driven by the first spool; and a second tower shaft engaged to bedriven by the second spool, wherein the one of the first tower shaft andthe second tower shaft drive one of the intermediate gears and the otherof the first tower shaft and the second tower shaft drive one of theinner armature or the outer armature.
 11. The turbofan engine as recitedin claim 10, wherein the drive means comprises an electric motor. 12.The turbofan engine as recited in claim 11, including a controller inelectric communication with the drive motor and the integrated motorgenerator for proportioning the first load and the second load betweenthe first spool and the second spool by controlling a speed of the drivemotor and stator fields of the inner armature and the outer armature.